Microsoft is currently testing the KB4512941 update for Windows 10 Version 1903 on Windows insiders. I reported about it in the blog post Windows 10 V1903: Updates KB4512941 and KB4515530. Now KB4512941 was updated. The the colleagues from deskmodder.de report here, the sandbox should finally work now. Some insiders are testing Microsoft's upgrade to Windows 10 19H2 (see Windows 10 Insider Preview Build 18363.327 (19H2)).